Intensity of Lights

Did you ever wonder about the intensity of lights on aids to navigation? Of course, we all would like to be able to see them clearly from a long distance. But, could they ever be too bright? Don't worry; there is a logical basis for the placement and intensity of lights for use by mariners. 

Lets start with the notion of a light being too bright. Imagine that a light marks the point of a shoal on a riverbank. If that light had the same intensity as the light on a lighthouse, it would look like a giant flash bulb as you passed by it at night.

Having that much intensity on a lighted buoy or dayboard would be detrimental to your night vision. However, when that same light intensity is on a lighthouse, 80 to 160 feet above the water level, it is not a problem. While on your vessel, you will never be "eye" level with the light on a lighthouse or in close proximity. It can be very bright and not have a negative impact on mariners.

The question is, how bright should a light be? The light's function is to guide mariners through a channel or a route toward their destination. The bottom line is that a light needs to be just bright enough to do its job. If it is any brighter than that, it may yield some unintended consequences.

An example would be a light on a buoy or dayboard that is one of several waypoints on a north/south route across a bay or sound. Assume that the waypoints immediately to the north and the south of the light are six nautical miles away. For the purpose of following that route, there is no need for the light to be visible for more than six nautical miles.

It may be a case that the same light is also the next waypoint for someone leaving the mouth of a nearby creek and heading for open water. In that case, the level of intensity considers that situation as well. But generally, lights have no more intensity than is needed.

Why do I bring this up? Because when you are out at night, your chart tells you how far you should be able to see a light in a certain state of visibility. In the description of a light on a chart, there is a reference that includes the capital letter "M" such as "6M" or "12M." Those references indicate that you can see those lights from distances of 6 and 12 nautical miles respectively.

The reference to 6 or 12 miles is based on conditions of 10-mile visibility. Your first question might be, how can you see a light from 12 miles away in 10-mile visibility. That's because 10-mile visibility is a standard of "clear" weather as defined by the International Visibility Code. It is actually a state of clarity, and not the maximum distance that you can see. 

Notations like 5M just let us know how far away we will be able to see a light. As the visibility increases or decreases, that distance increases or decreases. It is just one more piece of information to help us along the way.

Until next time, I wish you clear skies, fair winds, and calm seas!